Need Help Pulling Last Year's Data

I am writing a comparative financial statement comparing this year to last year this month to last years month. I need to write a formula that will pull this month for last year's data. The table/fields in my database are:

GLPerBal.FiscalPeriod = data is an integer, ie 1 = Jan, 2 = Feb etc
GLPerBal.FiscalYear = data is also an integer, 2,007 = 2007 etc

I have 2 parameters set up, {? Month}, {? Year}, to select the month and year.

This is the best I can come up with so far, and it doesn't work:

if {GLPerBal.FiscalYear} = ({? Year}-1)
and {GLPerBal.FiscalPeriod} = {? Month} then
{GLPerBal.CreditAmt} + {GLPerBal.DebitAmt}

I know this is a little complicated, but there are alot of experienced users on this site. Any help would be appreciated.

Thanks.

Is your record selection criteria selecting the information from last year? Or is it only selecting the information from this year? And are you entering this year as the year parameter or last year?

Where do you have the formula? It should be in the details section.

Hope this helps